1. Field of the Invention
The present invention relates to a semiconductor differential circuit used for a high-frequency circuit of, for example, a portable telephone, an oscillation apparatus, an amplifying apparatus and a switching apparatus using the semiconductor differential circuit, and a semiconductor differential circuit placement method.
2. Related Art of the Invention
With the rapid dissemination of portable telephones, it has become increasingly necessary to miniaturize the radio portion thereof. For that reason, it is a trend in recent years to render the radio portion as an IC. To embody the radio portion as an IC, it is necessary to embody as ICs an oscillator and a low noise amplifier which were manufactured with discrete parts and modules in the past.
FIG. 12 shows an example of a circuit embodied as an IC of an oscillation apparatus in the past. In the circuit shown in FIG. 12, a coil 1002 and a coil 1003 are connected in series, and a power supply 1001 is connected to a connection point of the coil 1002 and coil 1003. A series circuit of the coil 1002 and coil 1003 has a circuit to which switching elements 1006 and 1007 are connected in series via DC blocking capacitors 1004 and 1005 and a circuit to which variable capacity elements 1008 and 1009 are connected in series via DC blocking capacitors 1025 and 1026 connected in parallel thereto. A negative resistance circuit formed by MOSFETs 1010 and 1020 is connected to a resonant circuit formed as above so that the circuit as a whole is formed as a differential oscillator. The switching elements 1006 and 1007 are connected with a control voltage terminal 13 so that a frequency band of a calling frequency can be switched by a control voltage applied to the control voltage terminal 13.
FIG. 13 shows an example of a circuit embodied as an IC of an amplifying apparatus in the past. In the circuit shown in FIG. 13, a coil 1027 and a coil 1028 are connected in series, and a power supply 1029 is connected to the connection point of the coil 1027 and coil 1028. The series circuit of the coil 1027 and coil 1028 has MOSFETs 1030 and 1031 connected thereto so as to form a differential amplifier.
In the case where the oscillator shown in FIG. 12 is formed on a silicon substrate (not shown), however, a parasitic capacitance 1023 and a parasitic resistance 1024 are formed between it and the substrate on a drain 1021 side. Therefore, as shown in FIG. 14, the coil 1003 is equivalent to a parallel connection circuit to the series circuit of the parasitic capacitance 1023 and parasitic resistance 1024 as to a high-frequency signal component. Thus, if influenced by the parasitic capacitance 1023 and parasitic resistance 1024, a characteristic of the resonant circuit becomes a blunt one as shown by the broken line which was originally the one shown in full line as shown in FIG. 15. To be more specific, Q of the resonant circuit deteriorates and C/N deteriorates.
In the case where an amplifier circuit shown in FIG. 13 is formed on the silicon substrate, the parasitic capacitance 1023 and parasitic resistance 1024 are similarly formed between it and the substrate on the drains 1021 and 1022 side of the MOSFETs. Therefore, a high-frequency signal leaks to the parasitic resistance 1024 via the parasitic capacitance 1023. And a part of the high-frequency signal is lost due to influence of the parasitic resistance. Consequently, a noise characteristic deteriorates due to influence of a parasitic component of a gate, and a distortion characteristic deteriorates due to influence of the parasitic component of a drain.
In the case where a switching element is formed on the silicon substrate, the parasitic resistance and parasitic capacitance are formed as described above, consequently leading to a loss on turning on the switching element. When used in combination with the above oscillator for instance, in an ON state of the switching element, the resonant circuit is further influenced by the parasitic resistance and parasitic capacitance via the switching element. Thus, the Q-value further becomes blunt and the characteristic deteriorates.
To solve the above problems, some solutions have been provided. For instance, one solution proposes a constitution in which an oxide film is formed between a semiconductor device and the silicon substrate (refer to Japanese Translation of PCT International Application No. 11-501466 for instance). Thus, it is possible, by forming the oxide film between the semiconductor device and the silicon substrate, to reduce the parasitic capacitance 1023 so as to improve the characteristic deterioration of the oscillator and the low noise amplifier. In reality, however, a manufacturing process had to be changed to implement such a constitution, resulting in a high-cost process.
Another solution proposes a constitution in which impurity concentration of the silicon substrate is reduced to increase a resistance value of the parasitic resistance 1024 (refer to Japanese Patent Laid-Open No. 8-316420 for instance). The disclosures of Japanese Translation of PCT International Application No. 11-501466 and Japanese Patent Laid-Open No. 8-316420 are incorporated herein by reference in their entireties. FIG. 16 schematically shows the Q-value of the resonant circuit constituting the oscillator shown in FIG. 12 on changing the parasitic resistance 1024. Such a characteristic can be derived from a conductance and susceptance of the resonant circuit as the coils 1002 and 1003 in the circuit shown in FIG. 12 replaced by the circuit shown in FIG. 14. As for the characteristic shown in FIG. 16, the Q-value especially deteriorates in the range in which the parasitic resistance 1024 is 100 to 500 ohms. Therefore, to improve the Q-value of the resonant circuit, the resistance value of the parasitic resistance 1024 should be increased or decreased from the above range. Thus, it is possible, by reducing the impurity concentration of the silicon substrate, to increase the resistance value of the parasitic resistance 1024 so as to improve the characteristic of the oscillating circuit. It is also possible, in the amplifier circuit, to curb the characteristic deterioration by increasing the parasitic resistance. However, this solution also required the manufacturing process to be changed, resulting in a high-cost process.
A further solution proposes a contact for grounding the silicon substrate placed as close as possible to the MOSFETs. FIG. 17 is a plan view showing a configuration of such multi-finger type MOSFETs. In the configuration shown in FIG. 17, a source electrode 1032 in a longitudinal shape is placed, a gate electrode 1033 in a longitudinal shape is placed to be adjacent to the source electrode 1032, and a drain electrode 1034 in a longitudinal shape is placed to be adjacent to the gate electrode 1033. And a contactor 1035 connected to a silicon substrate wiring 1036 is placed close to the drain electrode 1034. The silicon substrate wiring 1036 is connected to an earth electrode. It is possible, by such a configuration, to reduce the resistance value of the parasitic resistance 1024 from the drain electrode 1034 to the earth electrode so as to improve the characteristic of the Q-value in the oscillating circuit for the above-mentioned reason. It is also possible, in the amplifier circuit, to curb the characteristic deterioration by reducing the parasitic resistance 1024.
As for the solution shown in FIG. 17, however, it is necessary to place a large number of contactors 1035 in order to sufficiently reduce a parasitic resistance 1024 in each drain electrode 1034. For instance, if MOSFETs 1010 and 1020 of the oscillating circuit shown in FIG. 12 are implemented on a semiconductor substrate, the placement will be as shown in FIG. 18. Thus, the area for mounting the contactors 1035 and silicon substrate wiring 1036 further becomes necessary so that the entire IC chip becomes large enough to be a factor of increased cost.
An object of the present invention is to provide a semiconductor differential circuit capable of miniaturizing an IC chip, an oscillation apparatus using the semiconductor differential circuit, an amplifying apparatus using the semiconductor differential circuit and a switching apparatus using the semiconductor differential circuit, and a semiconductor differential circuit placement method.